The present invention relates to a resist material sensitive to active beam irradiation, particularly a positive-working radiation-sensitive mixture.
The radiation-sensitive mixture is already known by itself. In particular, a positive-working mixture is commercially used as a resist material comprising, besides o-quinonediazide, a binder soluble in an aqueous alkaline solution, such as poly(4-hydroxystyrene) or novolac. However, this system is unsatisfactory in the sensitivity and resolution to radiation, particularly radiation in a short wavelength region. Novolac is unsuitable as a binder in a single layer resist material for deep UV (220-300 nm) because it has a high natural absorption in an UV-2 region (220-300 nm). On the other hand, poly(hydroxystyrene) (PHS) has more advantageous absorption properties in the UV region, a higher heat stability and a better dry etching resistance.
The positive-working radiation-sensitive mixture for UV-2, wherein PHS is used as a binder having a side group sensitive to an acid, is known from, for example, U.S. Pat. No. 4,491,628. It is also known that the sensitivity to radiation of the radiation-sensitive mixture can be enhanced by the addition of a compound which releases, upon exposure to radiation, an acid which has a catalytic action on the secondary reaction. Examples of the compound capable of forming a strong acid upon being exposed to radiation include a diazonium salt, a phosphonium salt, a sulfonium salt, an iodonium salt, a nitrobenzyl ester, a phenolic methanesulfonate, a diazo compound, a halogen compound, a bis-sulfonylmethane compound and a bis-sulfonyldiazomethane compound.
A positive-working radiation-sensitive mixture comprising a PHS copolymer, an acid generating compound and a N,O-acetal oligomolecule dissolution inhibitor is described by H. Roeschert et al. in "Critical Process Parameters of an acetal-based deep-UV photoresist", [Advances in Resist Technology and Processing IX, edited by Anthony E. Novembre, Proc. SPIE 1672, 33-45 (1992)]. The latent acid image of this photosensitive mixture is unfavorably unstable. The problem of the unstability of the acid latent image is a fundamental problem of new resist materials that function on the principle of a chemical amplification. This problem is fully discussed in technical literatures, for example, "Airborn chemical contamination of a chemically amplified resist", S. A. MacDonald et al., "Advance in Resist Technology and Processing VIII", edited by Hiroshi Itoh, Proc. SPIE 1466, 2-12 (1991). Further, studies by L. Schlegel show a problem of diffusion in chemically amplified resist materials (see L. Schlegel et al., Jap. Journ. of Applied Physics Series 5, Proc. of 1991 International Microprocess Conference, pp. 175-180). In a high-resolution resist for 248 nm, the average moving radius of the acid serving as a catalyst is the same as that of the size of the structure to be resolved. A method for determining the diffusion length and diffusion constant in chemically amplified resist materials is described by J. Nakamura in Jap. Journ. of Applied Physics (vol. 30, No. 10, October 1991, pp. 2619-2625). In a chemically amplified system very sensitive to a change in an acid latent image due to a low active energy barrier, the fundamental physical phenomenon limits the resolution. When no additive is used, the resists described by H. Roeschert et al. in [Advances in Resist Technology and Processing IX, edited by Anthony E. Novembre, Proc. SPIE 1672, 33-45 (1992)] have the following unfavorable properties due to the problem of diffusion.
1) The resolution is only down to 0.5 .mu.m. PA1 2) A high resolution can be achieved only by an unfavorable deviation from a linearity. PA1 3) The latitude of exposure is very small. PA1 4) The stability of the latent image due to holding time between the exposure and the subsequent baking after exposure is so low that the line slimming in a nonexposed resist region is remarkably large. PA1 a) a binder insoluble in water but soluble in an aqueous alkaline solution, PA1 b) a compound having at least one bond cleavable with an acid, PA1 c) a compound capable of producing an acid upon radiation, and PA1 d.sub.1) a basic ammonium compound. PA1 a) a binder insoluble in water but soluble in an aqueous alkaline solution, PA1 b) a compound having at least one bond cleavable with an acid, PA1 c) a compound capable of producing an acid upon radiation, and PA1 d.sub.1) a basic ammonium compound. PA1 1) The solubility in the resist is satisfactory. PA1 2) The heat stability is satisfactory. PA1 3) The basic counter ion is satisfactory. PA1 a) a binder insoluble in water but soluble in an aqueous alkaline solution, PA1 b) a compound having at least one bond cleavable with an acid, PA1 c) a compound capable of producing an acid upon radiation, and PA1 d.sub.2) a basic sulfonium compound. PA1 Y represents [CH.sub.2 ].sub.n wherein n is 0 or 1, O or S; PA1 R.sup.8 and R.sup.9 represent a C.sub.1 -C.sub.4 alkyl, alkoxy or a halogen; PA1 R.sup.10 and R.sup.11 represent a C.sub.1 -C.sub.4 alkyl, alkoxy or a halogen; PA1 n is 5 or 6; and PA1 X.sub.2 represents a basic anion having a pK.sub.B value of -3 to +5. PA1 1) The solubility in the resist is satisfactory. PA1 2) The heat stability is satisfactory. PA1 3) The basic counter ion is satisfactory. PA1 1) [Ph.sub.3 S].sup.+ (OH).sup.- +HO--aryl.fwdarw.[Ph.sub.3 S].sup.+- O-aryl PA1 2) [Ph.sub.3 S].sup.+ (OH).sup.- +HO--CO--R.fwdarw.[Ph.sub.3 S].sup.+- O--CO--R PA1 3) [Ph.sub.3 S].sup.+ (OH).sup.- +HO--SO.sub.2 --R.fwdarw.[Ph.sub.3 S].sup.+- O--SO.sub.2 --R PA1 1) Operation can be effected without involving a metal ions. PA1 2) A sulfonium salt, which is unstable in an isolated form, is produced and can be used in a radiation-sensitive mixture. PA1 3) A sulfonium salt, which cannot be produced in a pure form, is produced and can be used in a radiation-sensitive mixture. PA1 R.sup.14 represents a C.sub.1 -C.sub.10 alkyl or aryl group, PA1 Z represents a --CO--, --O--CO-- or --NH--CO-- group, and PA1 n is an integer more than 1.